Patent GB-859267-A (GB)
Title: Improvements in or relating to measuring apparatus
Inventor: BORLEY COLIN RODERICK
IPC: G01B7/02
Filed: 1959-06-25
Abstract: 859,267. Position-sensing apparatus. MULLARD Ltd. June 25, 1959, No. 21848/59. Class 40 (1).  Apparatus for measuring position or linear displacement comprises a bar having a series of regularly-spaced constant-pitch discontinuities, two detectors which can move along the bar at a fixed distance apart, a bridge circuit which provides zero output when the outputs of the detectors have a predetermined relationship (e.g. equal and opposite), but gives an error signal of appropriate sign when the detectors are in any other position, and discriminator means for rendering the indicator inoperative at or near alternate null positions. Preferably, the discontinuities in the measuring bar comprise a conductive helix and the detectors also have helical turns of the same pitch and sense and are capacitatively coupled to those of the measuring bar as in Specification 604,382, the separation of the detectors being an odd number of half-pitch lengths. A third detector, of the same type as the first two and rigidly connected to them at a spacing of an odd number of quarter-pitch lengths, is also capacitatively coupled to the measuring bar and serves to operate the discriminator means. In Fig. 2, the cylindrical measuring bar 2 and the three nut-shaped detectors 3, 4, 14 all have single-start helical threads of rectangular profile, the spacing of the threads being equal to their thickness (Fig. 1, not shown). An oscillator 11 supplies (e.g. at 25 kc/s.) a bridge circuit comprising detectors 3, 4 and the two halves of a centre-tapped transformer 10, whose output thus goes through maxima and minima as the detectors are moved along the bar. After detection at 12 a voltage is obtained which has similar variations and (with contact 21 as shown) is passed to the indicator, which therefore would show a series of nulls spaced by one thread pitch as the detectors move along the bar. A second bridge circuit includes a similar transformer 15 and the third detector 14 balanced against a constant capacitance 16, and owing to the relative spacing of the detectors the nulls in the output from 17 will coincide with maxima and minima from 12 and vice versa. By means of a squaring circuit and relay 20 the output of 12 is switched via another squaring circuit 18 during alternate half-cycles" of the waveform from 17 so that the indicator will not register a null but will be switched rapidly from one end of its scale to another. This use of alternate nulls avoids errors due to inaccuracies in the spacing of detectors 3, 4. The helical threads may be in the form of conducting surfaces on insulating members, and the detector nuts can be inside the measuring cylinder instead of outside. Multi-start threads can be used. The device is particularly suitable for positioning, or measuring the position of, the slide or table of a machine tool such as a jig-boring machine, and two similar devices can be used for the two co-ordinate directions. Either the bar or the detectors may be attached to the movable element. Fractional values can be obtained by rotating the detectors relative to the bar.